Based on the google maps acf example-code (and google maps api), I would suggest to change this:
// create marker
var marker = new google.maps.Marker({
position : latlng,
map : map
});
to something like this:
// create marker
var marker = new google.maps.Marker({
position : latlng,
map : map,
icon: ‘http://example.com/my_icon.png’
});
